While solar cells that are noticed as a clean energy source have been used for home electricity in recent years, the spread of the solar cell energy in the home has remained insufficient. The reasons are rather insufficient performance of the solar cell itself that renders a module to be of large size, low productivity of the module, and high price of the solar cell as a result of the drawbacks above.
The solar cell module is assembled by protecting a photoelectric conversion device such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, gallium-arsenic or copper-indium-selenium with an upper transparent protective material and lower substrate protective material, and the photoelectric conversion device and protective materials are fixed with a sealing agent to form a package. Accordingly, the sealing agent for the photoelectric conversion device of the solar cell is required to be excellent in a) moisture resistance, b) electrical insulation, c) heat resistance, d) moldability and workability, e) adhesive strength, f) purity, g) chemical resistance and h) gas-barrier property.
Today, an ethylene-vinyl acetate copolymer having a high content of vinyl acetate is used as the sealing agent for the photoelectric conversion device in the solar cell module from the viewpoint of flexibility and transparency (see Japanese Patent Application Laid-Open No. 11-54768). However, since heat resistance and adhesive property of the copolymer are not sufficient, an organic peroxide is required to be used together in order to complete the polymerization reaction. However, this required two production steps of forming a sheet of the ethylene-vinyl acetate copolymer containing such additives, and sealing the photoelectric conversion device using the sheet obtained. In the sheet production step of this process, extrusion molding speed cannot be increased since the sheet should be molded at a low temperature so that the organic peroxide is not decomposed. On the other hand, the sealing step of the photoelectric conversion device requires two time-consuming adhesion steps comprising a step for temporarily adhering in a laminator in several minutes to more than ten minutes, and a step for finally adhering in an oven in several tens of minutes to 1 hour at a temperature high enough for decomposing the organic peroxide. Although production of the solar cell module requires much labor and time as described above, adhesive property and reliability on moisture resistance remain not so improved.
Heat resistance is not sufficient when the copolymer or an ionomer having a low melting point is used (Japanese Patent Application Laid-Open No. 2000-186114). Such resins are not preferable since the module may be deformed by a temperature increase during the use of the photoelectric conversion device, or excess sealing agent may flow out when the solar cell module is produced by thermal compression bonding to form flashes. The stress applied on the seal portion in the production process has been remarkably increased in accordance with an increased size of photocells in recent years.
The sealing agent is required to be more reliable in moisture resistance corresponding to the increased length of seal lines. In addition, the sealing agent is also required to be able to equalize the thickness between conductive substrates used for the photoelectric conversion device from the view point of narrowing the seal lines, while the sealing agent is required to be excellent in adhesiveness and flexibility.
In 1991, a photo (solar) cell using a photoelectric conversion device named as a dye-sensitized solar cell was developed by Graetzel (Switzerland). This solar cell, which is also called as a Graetzel cell, comprises a charge transfer layer (an electrolyte solution containing a redox substance) interposed between a thin film substrate, which comprises oxide semiconductor particles that is sensitized with a dye and serves as one electrode on a transparent conductive substrate, and a substrate, facing the transparent conductive substrate, that serves as a counter-electrode on which a reducing agent such as platinum is disposed.
For sealing an electrolyte solution between both electrodes of the dye-sensitized solar cell, square columns made of a glass, metal or plastic as sealing solids are bonded with an epoxy resin or a silicone resin between the electrodes (see Japanese Patent Application Laid-Open No. 2000-173680). However, work process becomes complicated in such composite sealing process. Moreover, water vapor permeability is large when a silicone resin is used for sealing, and this method is not suitable for long term sealing of a liquid such as the electrolyte solution in the dye-sensitized solar cell.
Use of a polyisobutene resin as a sealing agent of the electrolyte solution for the dye-sensitized solar cell is also reported (Japanese Patent Application Laid-Open No. 2002-313443) According to this report, a thermosetting resin is used in primary sealing for bonding both electrodes, and a UV-curable resin is used in secondary sealing for blocking an injection port after injecting an electrolyte solution through the port. However, the resin cannot be sufficiently hardened since the sealing agent contacts the redox charge transfer layer. While water vapor permeability of this resin is quite low as compared with a silicone resin, its ductility is so large that changes of its property against temperature changes are too large. Adhesive strength of the resin at a low temperature is low with additional drawbacks of poor workability, poor abrasion resistance and slow elastic recovery to render the solar cell to have poor durability for long term uses.
As hitherto described, the sealing agents that have been proposed cannot satisfy all the performances required as a sealing agent for the photoelectric conversion device, particularly for the photoelectric conversion device for the dye-sensitized solar cell. In particular, the conventional sealing agent has no performance for sealing the redox charge transfer layer.